1. Field of the Invention
This invention relates to a base plate for magnetic recording disc having no defect.
2. Description of the Prior Art
Heretofore, as base plates for magnetic discs with sputtered recording media, there have been used those produced by anodizing treatment on surfaces of aluminum alloy substrate because of its light-weight, followed by abrasion or polishing. The anodized film is formed for improving abrasion workability by increasing hardness of the surfaces of the aluminum substrate and for preventing diffusion or chemical reaction between the aluminum alloy substrate and magnetic recording media such as iron oxide formed on the substrate. But the base plates for magnetic discs produced by the above-mentioned process have disadvantages in that in the case of using an aluminum alloy as a substrate, since intermetallic compounds are produced from trace amounts of iron, manganese, silicon, etc., included in the aluminum alloy substrate so as to be precipitated on crystal grain boundaries or in crystal grains, pin holes having a diameter of 3 .mu.m or more are produced on the portions where the intermetallic compounds are precipitated on the anodized film. For example, when the aluminum alloy substrate contains iron in an amount of at least 0.005% by weight, intermetallic compounds are precipitated to form defects on the anodized film. In order to remove such a disadvantage, it is thinkable to reduce the amounts of impurities in the substrate, but such a thing can hardly be attained practically, and even if the impurity amounts can be reduced to such an extent, the cost of substrate becomes disadvantageously expensive.
Another method proposed for reducing the number of pin holes on the anodized film is to form layers of pure aluminum and the like on the surfaces of the substrate by a rolling process, or a cladding process. But according to this cladding process, there is another disadvantage in that impurities are transferred to the cladding layer from rolling rollers and a lubricant at the time of rolling and many defects are formed on an anodized film formed on the cladding layer. Further, there is proposed a process for adhering a plastic to the substrate mentioned above by using an adhesive or by contact bonding with heating. But such a process has a disadvantage in that the bonding layer is not even. According to a process wherein the above-mentioned substrate is formed by molding a plastic, strain is produced in the molded portions and cracks are produced unfavorably during the use.
On the other hand, there is proposed a process wherein pin holes on the anodized film are filled with a thermosetting resin, followed by polish finish, but the number of pin holes cannot be reduced remarkably according to this process.
Further, according to a process wherein a base plate for magnetic recording disc is produced by forming a film of oxide such as SiO.sub.2, Ta.sub.2 O.sub.5, Al.sub.2 O.sub.3 or the like by a sputtering process or the like directly on an aluminum alloy substrate, adhesiveness of the oxide film to the substrate is unfavorably not good.